Why is my PO Approval Workflow: Send E-mail Inconsistent?

I created a Purchase Order approval workflow and is working fine except for the send e-mail action that is not working consistently. When an employee creates a PO that is above their purchasing limit, the PO approval status is set to Pending Approval and the next approver receives an email. However, when the next approver, approves the PO, the second next approver does not get a notification email. When the final approver approves, the employee who created the Purchase Order gets an email that the PO has been approved. In a nutshell, all secondary approvers do not get a notification